This invention relates to plating processes and, more particularly, to selected plating processes each of which utilizes directed beams of high intensity acoustic waves to create non-linear effects that alter and improve each of the plating processes.
The plating of objects, such as circuit boards, may be accomplished by various processes. One such process is an immersion type of selective electroplating that involves a tank filled with plating solution or electrolyte. Electrodes are placed in the tank, sometimes referred to as a plating bath, and a voltage is applied to the electrodes to set up an electric field in the tank. A power supply that has a variable voltage and amperage control is used to create the voltage between the electrodes. One electrode, called the anode, is attached to the positive terminal of a power supply. The other electrode, called the cathode, is attached to the negative or ground terminal of the power supply. The object to be plated is connected to the negative side of the power supply and, in effect, the object becomes the cathode side of the circuit. The plating material will deposit on the cathode side of the circuit.
The electrolyte consists of a chemical solution where the plating material, usually a metal, is dissolved and suspended in the liquid as metal ions. When the electric field is applied, the metal ions deposit on the cathode increase in thickness over time until the process is halted or the solution is exhausted. If the liquid is quiescent with no liquid currents or agitation, the electrolyte in the liquid layer immediately adjacent to the plated surface will become depleted of ions. The rate of metal ion deposition will slow down by an order of magnitude unless the electrolyte is agitated or some other means is used to deliver fresh electrolyte. Agitation is also used to dislodge and sweep away bubbles that sometimes form on the plating surface. The bubbles, typically hydrogen, form as a consequence of the deposition process and the bubbles will obstruct plating and create pinholes and non-uniform thickness in the plating.
Uneven distribution of plating is most often due to localized variations of electric field intensity. The variation is further exacerbated when objects have features with sharp edges or thin conductive paths created during the plating process and coexisting with large planar areas of the objects being plated. Plating in the large areas of the objects may be a fraction of the thickness of that on the narrow paths of the objects. Manipulation of the anode position and variations in anode shape are used to minimize the effect by redistributing the electric fields. Special tanks and customized anodes may be needed which disadvantageously raises the tooling cost to perform the plating. For complex objects, rotation assemblies and complex tumbling schemes may be required. In many cases the simplest solution is to plate the object until the thinnest area meets the minimum thickness required. This means that excess material will be applied to most other locations. This practice is particularly costly for precious metal plating.
Another disadvantage occurring in prior art electroplating is that as plating takes place the plating solution is continuously creating concentration gradients. For simple flat objects agitation works well. For complex objects with isolated cavities simple agitation cannot provide complete uniformity. Another disadvantage is that the formation of hydrogen or oxygen gas is a byproduct of the process. Again, agitation helps suppress formation and dislodge bubbles. Unfortunately, for complex objects the bubbles dislodged in one location may be trapped in another location.
A xe2x80x9cmaskxe2x80x9d is typically used in electroplating to cover surface areas that are not intended to be plated. The mask may be a simple water-proof adhesive tape applied manually. It may also be a solid rubber mask held in place mechanically. Most commonly, a photolithographic technique is used where a light sensitive liquid hydrocarbon, called a photoresist, is applied as a coating. A photo tool similar to a photographic negative is used to define the shape of the mask by blocking light in some areas and transmitting light in others. The photoresist liquid solidifies when it is exposed to light while the unexposed photoresist remains liquid. The unsolidified liquid is removed and the remaining solid photoresist forms a mask that protects the coated areas from the plating process.
A simple tape mask is applied manually and can be very labor intensive, but suitable for low volume production or repairs. The typical photolithographic or photomask is suitable for high volume but requires a photo tool to be used which is costly to produce. If a change to the pattern is needed, the phototool is discarded and a new one is made. Some masking chemicals are toxic, carcinogenic or create compounds that are toxic. The materials are applied, exposed and developed and later stripped. Each stage creates waste that may be toxic and poses special handling and disposal costs. The light sensitivity of the photomasks, like film, implies special handling and storage facilities. The photomasks also have a limited shelf life. The material may be wasted even before it is used due to inadvertent exposure to light or heat and the simple degradation in storage. The new aqueous materials are more acceptable from a health consideration but they are not as sturdy as hydrocarbon types. For example, these aqueous masks are resistant to acid solutions but are dissolved by basic solutions thus, there is an inherent sensitivity to plating pH value. When the plating solution deposits the metal, it also becomes more basic and in some cases will attack the photomask and ultimately result in a flawed or damaged product.
The electroplating process may be enhanced by a periodic reversal of the plating voltage known as a xe2x80x9cpulse platingxe2x80x9d process in the prior art. The objective is to periodically reverse the voltage of the plating process so that the metal previously deposited is now extracted. The extraction rate is highest around features of objects being plated that have sharp or narrow geometry where electric fields are strong due to edge effects. The high extraction rate counteracts the excessive deposits caused by the same edge effects during the plating process. One of the main disadvantages of pulse plating is that it only reverses the process and there is a slight difference between deposition rate and extraction rate which may lead to non-uniform thickness of the plating.
Another process for performing plating is autocatalytic plating, most often called xe2x80x9celectroless platingxe2x80x9d, and may be used to plate nonconductive surfaces where electroplating does not work well. The electroless plating does not rely on electric fields and therefore is harder to regulate. Typically this process may be used for plating nonconductive surfaces such as plastics and ceramics. The electroless plating is typically used to plate the holes in circuit boards so as to provide a conductive electrical path thereon. In some applications of electroless plating, a base of conductive metal plating is applied by electroless means and then the object is moved to an electroplating process for greater speed and control. Masking is somewhat critical for selective electroless plating because this process does not depend on the pattern of the conductive substrate to define the deposit pattern thereon. Since electroless plating does not rely on externally applied electric fields, it is harder for the user to regulate the process. Unlike electroplating the electroless plating process cannot be easily turned xe2x80x9cONxe2x80x9d and xe2x80x9cOFFxe2x80x9d by application of electric power. For objects, such as electronic circuit boards, the failure of a mask will cause insulator surfaces to become conductive which may disadvantageously lead to short circuits.
A further process used to perform plating is brush plating. Brush plating is a selective electroplating process that is usually done by hand and thus is used mostly for repairs, very low production volume or plating in the field due to the limited provisions needed. A xe2x80x9cbrushxe2x80x9d is a hand held anode with a porous wrap of cloth, sponge, cotton or other material that may be swabbed or brushed along a surface. Like immersion plating the process will place a positive voltage on the brush and a negative or ground on the object being plated. The brush may be a simple swab saturated liquid electrolyte or it may be connected to a pumped electrolyte supply. The advantages of brush plating are the low cost of equipment, little or no masking, and the ability to make quick repairs. However, this process is prone to spills and brings the user in close contact with corrosive, reactive and even toxic materials. The manual application disadvantageously requires a skill to achieve uniform and consistent results and also masking is often required.
A still further process used for performing plating is jet plating. Jet plating uses a liquid jet emitted by nozzles and aimed at a target object. A jet is a continuous free stream where the liquid remains continuous until it impinges on the part. The continuous stream will support an electric current and thus can be used for electroplating. Plating occurs only in the areas wetted by the jet of liquid. Because of the difficulty controlling the behavior of the liquid once it contacts the surface, the process normally requires a mask for selective plating. Jet spraying that creates the free stream jet of liquid plating solution undesirably requires pumps and nozzles that resist corrosive and reactive chemicals. This process also disadvantageously requires masking for selective plating.
An additional process used to perform plating is spray plating. Spray plating uses a nozzle to atomize or break a liquid stream into droplets that then impinge on the object to be plated. The isolated droplets may be charged, but will not sustain a continuous current between the source and the object. This process is generally used for electroless plating. The process has the advantage of controlling the distribution and thickness in a manner similar to spray painting used, for example, for automobiles. However, masking is disadvantageously required for selective plating. Further, the atomization process creates a mist of toxic, corrosive or reactive material that may coat other objects and may be inhaled by workers.
It is desired that a plating process be provided that does not suffer the drawbacks of the prior art plating processes, while at the same time providing improvements over each of the prior art plating processes.
The invention is directed to a system and a method for improving selective plating processes by using direct beams of high intensity acoustic waves to create non-linear effects for improving the plating process. The invention improves selective plating of a surface on an object which in one embodiment is immersed in a plating solution, and in another embodiment is suspended above the plating solution.
The system comprises a container, a source of radio frequency (RF), and a transducer. The container holds the plating solution in which is lodged an anode and cathode electrode connected to respective electroplating voltages and with the cathode electrode connected to the object to be plated. The container has means for holding an object at least near or submerged in the plating solution. The transducer receives the output of the RF source and develops an output beam of acoustic waves that are directed at least near the object.